High power distributed feedback ridge waveguide laser

ABSTRACT

A distributed feedback ridge waveguide semiconductor laser diode having a waveguide region with a typical thickness of at least 500 nanometers and an effective refractive index difference between the ridge structure and exposed portions of the waveguide region which surround the ridge structure of less than 0.001. This permits the width of the ridge to be expanded beyond 3.5 microns thus translating directly to higher power outputs at 1.55 μm wavelengths, where carrier diffusion and carrier heating limit current density injected into the active region.

PROVISIONAL APPLICATION

[0001] This application claims the benefit of Provisional application60/176,915 filed Jan. 20, 2000.

FIELD OF THE INVENTION

[0002] The present invention relates to a ridge waveguide (RWG)semiconductor laser diode having increased output power, to adistributed feedback (DFB) RWG semiconductor laser diode of this kindwhich exhibits dynamic single longitudinal mode along with increasedoutput power, and, more particularly, to a high power RWG semiconductorlaser diode, such as a DFB RWG semiconductor laser diode, having reducedantiguiding effects within the waveguide which permits a larger singlemode guide to be utilized.

BACKGROUND OF THE INVENTION

[0003] High efficiency, high power lasers have long been pursued forsuch applications as optical pumping of solid state and fiber lasers,direct material processing, printing, communications, sensing, etc. Forexample, U.S. Pat. No. 5,818,860, entitled High Power SemiconductorLaser Diode, assigned to David Samoff Research Center, Inc., describes abroadened-waveguide technique for producing high-power DFB lasers.

[0004] The broadened waveguide concept described in U.S. Pat. No.5,818,860 permits low loss and therefore high-power lasing in multimodesources. Other characteristics inherent in this concept are ofparticular promise for high-power single-spatial-mode anddynamic-single-longitudinal-mode lasing. Results of an initial attemptin which the broadened waveguide was incorporated into a 1.55 μm singlemode DFB RWG diode laser has provided encouragement; as there wasattained a 200 mW power output single mode, −165 dBm/Hz RIN from 0 to 2GHz, and 200 kHz linewidths for 1.5 mm cavity length implementations.Further, FIG. 1 shows the RIN performance achieved and 300 MHz linewidthwith a broadened waveguide DFB laser. As shown in FIG. 2, this laseremitted 200 mW cw at 1.55 μm wavelength.

[0005] High-power ridge waveguide (RWG) lasers use a cold-cavity index,i.e., effective index, stepof ˜Δn=0.01, but this value under currentinjection is diminished by antiguiding. Although antiguiding isquantitatively difficult to estimate accurately and is variable,proprietary experiments and extensive published accounts of conventionalRWG laser structures lead one to conclude that latitude in the choice ofΔn is severely compromised by the antiguiding phenomenon. As a result,Δn must be designed to substantially exceed the maximum anticipatedantiguiding diminution. For RWG lasers of the prior art, antiguiding hasrequired Δn values so great that ridge widths must be limited to ˜3.5 μmor narrower to attain a stable, single waveguide mode. The restrictionin ridge width limits the power that can be achieved by the laser forseveral reasons: Firstly, the maximum current density that can beusefully pumped into a semiconductor active region may be limited byphenomena such as the maximum attainable conduction band offsets orother phenomena which affect the maximum power attainable. For widerridges, a greater current per unit length can usefully be pumped intothe active region, causing higher powers to be emitted. Such effectslimit the maximum power emitted by the RWG laser under both cw andpulsed conditions. Secondly, an increased ridge width would provide agreater surface area for heat dissipation. Since laser diode performanceis severely restricted as temperature rises, wider ridges would permitgreater currents to be pumped into the RWG laser and greater powers tobe emitted. Such effects presently limit the maximum power emitted by aRWG laser under cw conditions

[0006] Therefore, a high-power RWG laser having a ridge width greaterthan ˜3.5 μm is needed to provide further gains in power output from anyRWG laser such as a DFB RWG laser

SUMMARY OF THE INVENTION

[0007] A semiconductor laser diode comprises a body of a semiconductormaterial having a length of at least substantially 3 millimeters; alow-propagation-loss waveguide region formed in the body, having athickness of at least 500 nanometers; a ridge structure disposed over aside of the waveguide region. For applications requiring dynamicsingle-longitudinal-mode operation, the diode also includes adistributed feedback structure associated with at least one of thewaveguide region and ridge structure. The effective refractive indexdifference between the ridge structure and exposed portions of thewaveguide region which surround the ridge structure is less than 0.003.Accordingly, the width of the ridge can be expanded beyond 3.5 microns.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The advantages, nature, and various additional features of theinvention will appear more fully upon consideration of the illustrativeembodiments now to be described in detail in connection withaccompanying drawings wherein:

[0009]FIG. 1 is a graph plotting linewidth vs. power output of a priorart broadened waveguide DFB laser;

[0010]FIG. 2 is a graph plotting power output vs. injection current of aprior art broadened waveguide DFB laser; and

[0011]FIG. 3 is a perspective view of a DFB RWG semiconductor laserdiode according to an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0012] The high-power DFB RWG laser of the present invention set forthin greater detail further on employs the broadened-waveguide technologydescribed in U.S. Pat. No. 5,818,860, the entire disclosure of which isincorporated herein by reference, to expand the ridge width of the laserbeyond 3.5 μm. The reduced propagation losses due to reduced modaloverlap with doped regions permits the laser to operate with reducedoverlap with the quantum wells, as occurs in the broadened-waveguidelaser of the '860 patent, and causes the active region to operate withlower gain (and lower carrier concentration)which concomitantly reducesantiguiding. This is because it is the nature of the antiguiding effectthat a reduction in index is proportional to the increase in the opticalgain per unit length, as compared to the materials surrounding theridge. This effect permits a smaller effective refractive index or indexdifference Dn between the ridge region and the surrounding etchedregion, in turn permitting wider single mode ridge waveguides. Thistranslates directly to higher power outputs at wavelengths such as 1.55mm, where carrier diffusion and carrier heating limit current densitywhich can usefully injected into the active region, particularly inInGaAsP laser.

[0013] Turning now to FIG. 3, there is shown a DFB RWG semiconductorlaser diode 10 according to an exemplary embodiment of the presentinvention. The laser diode 10 comprises a body 12 of a semiconductormaterial or materials having a bottom surface 14, top surface 16, endsurfaces 18 and side surfaces 20. The body 12 includes a waveguideregion 22 extending thereacross. Within the waveguide region 22 is anactive region 24 in which photons are generated when an appropriateelectrical bias is placed across the diode 10. The active region 24 maybe of any structure well known in the laser diode art which is capableof generating photons consistent with the requirement of attaining lowoptical propagation losses through a broadened waveguide design, orequivalent. Preferably, the active region 24 comprises one or morequantum wells. The waveguide region 22 includes a first layer 25 of“undoped” semiconductor material on a first side of the active region 24and a second layer 26 of “undoped” semiconductor material on a secondside of the active region 24. The first and second layers 25 and 26 ofundoped semiconductor material have a doping level of no greater thanabout 5×10¹⁶ atoms/cm³.

[0014] A first clad region 28 is disposed on the first side of thewaveguide region 22. The first clad region 28 may be composed of asemiconductor material of a P-type conductivity. A second clad region30, which may be formed of a N-type conductivity, is disposed on thesecond side of the waveguide region. The first clad region 28 is etchedso as expose portions of the underlying first layer 25 of undopedsemiconductor material. The etched clad region 28 defines a ridge-likestructure 31 having a width W. For dynamically single longitudinal modeoperation, a distributed feedback structure, formed by corrugations 33,is etched in either the ridge-shape first clad region 28 as shown or inthe first layer 25 of undoped semiconductor material.

[0015] The composition of the first and second clad regions 28 and 30 ofa semiconductor material is of a lower refractive index than thematerials of the first and second layers 25 and 26 of the waveguideregion 22. The doping level in the first and second clad regions 28 and30 are typically between about 5×10¹⁷ atoms/cm³ and 2×10¹⁹ atoms/cm³.

[0016] A contact layer 32 of a conductive material, such as a metal, ison and in ohmic contact with the P-type conductivity ridge-shaped cladregion 28. The contact layer 32 is in the form of a strip which extendsbetween the end surfaces 18 of the body 12 and may be narrower than thewidth of the body 12, i.e., the distance between the side surfaces 20 ofthe body 12. A contact layer 34 of a conductive material, such as ametal, is on and in ohmic contact with the N-type conductivity cladregion 30. The contact layer 34 extends across the entire area of thebottom surface 14 of the body 12.

[0017] The thickness of the waveguide region 22 and the composition ofthe waveguide region 22 and the clad regions 28 and 30 must be such thatthe optical mode generated by the active region 24 does not overlap fromthe waveguide region 22 into the more heavily doped clad regions 28 and30 by more than 5%, and preferably by not more than 2%. However, theamount of overlap of the photons into the clad regions 28 and 30 neednot be less than 1%. This means that the amount of the optical mode,which is mainly in the waveguide region 22, that extends into (overlaps)the clad regions 28 and 30 is no greater than about 5% of the totaloptical mode. To achieve this, the thickness of the waveguide regionshould be at least 500 nanometers (nm) and the composition of thewaveguide region 22 and the clad regions 28 and 30 should be such thatthe refractive index of the regions provides the confinement of theoptical mode in the waveguide region 22 to the extent that the overlapof the optical mode into the clad regions 28 and 30 is not greater than5%. The various regions of the body 12 may be made of any of the wellknown semiconductor materials used for making laser diode, such as butnot limited to gallium arsenide, aluminum gallium arsenide, indiumphosphide, indium gallium arsenide and such quaternary materials asindium, gallium arsenide phosphide. However, the materials used for thevarious regions must have refractive indices which provide the desiredconfinement of the optical mode. The clad regions 28 and 30 may be dopeduniformly throughout their thickness or may be graded with little or nodoping at their junction with the waveguide region 22 and the heaviestdoping at the respective surface of the body 12.

[0018] The laser diode 10 of the present invention can be made longerthan conventional laser diodes, i.e., in lengths of substantially 3millimeters or longer, because there is lower optical propagation lossin the laser diode of the invention. Moreover, the broadened waveguideregion 22, with its reduced antiguiding effects, enables the ridgestructure 31 to be etched so that the effective refractive index Δn,i.e., index difference, between the ridge structure 31 and the exposedportions of the underlying first layer 25 of undoped semiconductormaterial surrounding the ridge structure 31 is substantially reduced tobetween about 0.0007 and 0.002. This, in turn, permits width W of theridge to be increased substantially beyond the 3.5 μm widths ofconventional designs to widths of 5 μm and greater, thereforetranslating directly to higher power outputs per unit length at 1.55 μmwavelengths, where carrier diffusion and carrier heating limit currentdensity injected into the active region, particularly in InGaAsP. Thatis, power increases due to increased length of the diode as taught inthe '860 patent are further extended by 50% to 100% in the structuretaught here by increase of the ridge width for an index-guided RWG lasersuch as a RWG DFB laser diode.

[0019] Additionally, the distributed feedback structure produces acoupling constant κ which is about 3 times greater than similarstructures in conventional laser diodes because of the 3 times greaterwidth of the ridge structure. Consequently, further improvements inthermal dissipation and power density are realized with the laserstructure of the present invention.

[0020] While the foregoing invention has been described with referenceto the above embodiments, various modifications and changes can be madewithout departing from the spirit of the invention. Accordingly, allsuch modifications and changes are considered to be within the scope ofthe appended claims.

What is claimed is:
 1. A semiconductor laser diode comprising: a body ofa semiconductor material having a length of at least 2.5 millimeters; awaveguide region formed in the body, the waveguide region includingactive region for generating an optical mode of photons, the waveguideregion having a thickness which supports a mode exhibiting a 5% or lessoverlap with a highly doped p-clad layer; a ridge structure disposedover a side of the waveguide region; and wherein the effectiverefractive index difference between the ridge structure and exposedportions of the waveguide region which surround the ridge structure isless than 0.002.
 2. The semiconductor laser diode of claim wherein adistributed feedback structure is associated with at least one of thewaveguide region and ridge structure
 3. The semiconductor laser diode ofclaim 1 wherein the waveguide region has a doping level of no greaterthan 5×10¹⁶ atoms /cm³.
 4. The semiconductor laser diode of claim 1wherein the ridge structure is defined by one of two clad regionsdisposed on opposing sides of the waveguide region, the clad regionsbeing at least partially doped to be of opposite conductivity types. 5.The semiconductor laser diode of claim 3 wherein the materials of thewaveguide region and the clad regions have a refractive index whichprovides confinement of the optical mode to the waveguide region with anoverlap of the optical mode into the clad regions of no greater than 5%.6. The semiconductor laser diode of claim 3 wherein the clad regions areof a semiconductor material having a lower index of refraction than thematerials of the portions of the waveguide region adjacent the cladregions.
 7. The semiconductor laser diode of claim 3 wherein the ridgestructure has a width that is 5 microns or greater.
 8. The semiconductorlaser diode of claim 1 wherein the distributed feedback structurecomprises corrugations.
 9. The semiconductor laser diode of claim 1wherein the distributed feedback structure is formed in the ridgestructure.
 10. The semiconductor laser diode of claim 1 wherein thedistributed feedback structure is formed in the waveguide region. 11.The semiconductor laser diode of claim 1 wherein at least a portion ofthe body is made from a semiconductor material selected from the groupconsisting of gallium arsenide, aluminum gallium arsenide, indiumphosphide, indium gallium arsenide and indium, gallium arsenidephosphide.
 12. The semiconductor laser diode of claim 1 wherein theridge structure has a P-type conductivity.
 13. A semiconductor laserdiode comprising: a body of a semiconductor material having a length ofat least 3 millimeters; a waveguide region formed in the body, thewaveguide region including active region for generating an optical modeof photons, the waveguide region having a thickness which supports amode exhibiting a 5% or less overlap with a highly doped p-clad layer; aridge structure disposed over a side of the waveguide region; andwherein the ridge structure has a width that is greater than 3.5microns.
 14. The semiconductor laser diode of claim 13 wherein adistributed feedback structure is associated with at least one of thewaveguide region and ridge structure;
 15. The semiconductor laser diodeof claim 13 wherein the effective refractive index difference betweenthe ridge structure and exposed portions of the waveguide region whichsurround the ridge structure is less than 0.002.
 16. The semiconductorlaser diode of claim 13 wherein the waveguide region has a doping levelof no greater than 5×10¹⁶ atoms /cm³.
 17. The semiconductor laser diodeof claim 13 wherein the ridge structure is defined by one of two cladregions disposed on opposing sides of the waveguide region, the cladregions being at least partially doped to be of opposite conductivitytypes.
 18. The semiconductor laser diode of claim 17 wherein thematerials of the waveguide region and the clad regions have a refractiveindex which provides confinement of the optical mode to the waveguideregion with an overlap of the optical mode into the clad regions of nogreater than 5%.
 19. The semiconductor laser diode of claim 17 whereinthe clad regions are of a semiconductor material having a lower index ofrefraction than the materials of the portions of the waveguide regionadjacent the clad regions.
 20. The semiconductor laser diode of claim 13wherein the distributed feedback structure comprise corrugations. 21.The semiconductor laser diode of claim 13 wherein the distributedfeedback structure is formed in the ridge structure.
 22. Thesemiconductor laser diode of claim 13 wherein the distributed feedbackstructure is formed in the waveguide region.
 23. The semiconductor laserdiode of claim 13 wherein at least a portion of the body is made from asemiconductor material selected from the group consisting of galliumarsenide, aluminum gallium arsenide, indium phosphide, indium galliumarsenide and indium, gallium arsenide phosphide.
 24. The semiconductorlaser diode of claim 13 wherein the ridge structure has a P-typeconductivity.